Liquid crystalline medium and liquid crystal display

ABSTRACT

The instant invention relates to liquid crystalline media comprising
         a chiral component, component A, consisting of one or more chiral compounds,   optionally, a bimesogenic component, component B, consisting of one or more bimesogenic compounds,   a liquid crystalline component, component C, consisting of one or more liquid crystalline, respectively mesogenic compounds, and   a reactive mesogenic component, component D, comprising, one or more reactive mesogenic compounds,
 
as defined in claim  1 , to their stabilization by polymerization and to the polymer-stabilized liquid crystal materials, as well as to liquid crystal displays comprising these liquid crystal media, respectively these stabilized materials, especially to USH-displays and in particular to active matrix displays and, last not least, to the processes of preparation of the respective composite systems and of the displays comprising these systems.

FIELD OF THE INVENTION

The present invention relates to liquid crystalline media, preferably tocholesteric (chiral doped nematic) media, comprising one or morenon-reactive liquid crystalline compounds, one or more chiral dopants,optionally one or more bimesogenic compounds and one or more reactivemesogenic compounds, respectively, to such liquid crystalline mediacomprising one or more non-reactive liquid crystalline compounds, one ormore chiral dopants and optionally one or more bimesogenic compounds,stabilized by a polymer and to liquid crystal displays comprising thesemedia, especially to displays operating in the USH mode and inparticular to displays addressed by an active matrix.

STATE OF THE ART AND PROBLEM TO BE SOLVED

Liquid Crystal Displays (LCDs) are widely used to display information.LCDs are used for direct view displays, as well as for projection typedisplays. The electro-optical mode which is employed for most displaysstill is the twisted nematic (TN)-mode with its various modifications.Besides this mode, the super twisted nematic (STN)-mode and morerecently the optically compensated bend (OCB)-mode and the electricallycontrolled birefringence (ECB)-mode with their various modifications, ase.g. the vertically aligned nematic (VAN), the patterned ITO verticallyaligned nematic (PVA)-, the polymer stabilized vertically alignednematic (PSVA)-mode and the multi domain vertically aligned nematic(MVA)-mode, as well as others, have been increasingly used. All thesemodes use an electrical field, which is substantially perpendicular tothe substrates, respectively to the liquid crystal layer. Besides thesemodes there are also electro-optical modes employing an electrical fieldsubstantially parallel to the substrates, respectively the liquidcrystal layer, like e.g. the In Plane Switching (short IPS) mode (asdisclosed e.g. in DE 40 00 451 and EP 0 588 568) and the Fringe FieldSwitching (FFS) mode. Especially the latter mentioned electro-opticalmodes, which have good viewing angle properties and improved responsetimes, are increasingly used for LCDs for modern desktop monitors andeven for displays for TV and for multi media applications and thus arecompeting with the TN-LCDs.

Further to these displays, new display modes using cholesteric liquidcrystals having a relatively short cholesteric pitch have been proposedfor use in displays exploiting the so called “flexo-electric” effect. Inthese displays the cholesteric liquid crystals are oriented in the“uniformly lying helix” arrangement (ULH), which also give this displaymode its name. In this mode, however several problems still have to beresolved, which are, amongst others, difficulties in obtaining therequired uniform orientation, an unfavourably high voltage required foraddressing, which is incompatible with common driving electronics, a notreally dark “off state”, which deteriorates the contrast, and, last notleast, a pronounced hysteresis in the electro-optical characteristics.

A relatively new display mode, the so-called uniformly standing helix(USH) mode, may be considered as an alternative mode to succeed the IPS,as it can show improved black levels, even compared to other displaymode providing wide viewing angles (e.g. IPS, VA etc.).

For the USH mode, like for the ULH mode, flexo-electric switching hasproposed, using bimesogenic liquid crystal materials. However, due tothe unfavourably high driving voltage required, to the relatively narrowphase range of the chiral nematic materials and to their irreversibleswitching properties, these materials are not compatible for used withcurrent LCD driving schemes.

Surprisingly it has now been found that LCDs operating in the USH modeusing calamitic LC material, having high values of the dielectricanisotropy (Δ∈) allows to apply dielectric switching as an alternativeconcept to flexoelectric switching to overcome above mentioneddifficulties. Furthermore, it has been found that polymer stabilizationof the materials, preferably using RM materials is very beneficial forthe recovery of initial black state of the displays.

The liquid crystals (LCs) according to the present invention arepreferably used in improved LCDs using cholesteric liquid crystals,which are also known as chiral nematic liquid crystals, with shorthelical pitch and with high dielectric anisotropy especially foradvanced applications. They are particularly useful for operation inreflected mode, as cholesteric liquid crystals having an appropriatecholesteric pitch selectively reflect light they may be coloured andallow to avoid the use of colour filters in LCDs.

For these applications new liquid crystalline media with improvedproperties are required. Thus liquid crystalline media with improvedbehaviour are required. Their rotational viscosity should be as low aspossible. Besides this parameter, the media have to exhibit a suitablywide range of the nematic phase, respectively the cholesteric phase, anappropriate birefringence (Δn), preferably in the range from 0.100 to0.300 and a suitably high dielectric anisotropy (Δ∈). Δ∈ has to besufficiently high to allow a reasonably low operation voltage.Preferably Δ∈ should be 20 or more, more preferably 30 or more, in orderto allow use easy accessible drivers with reasonably low operationvoltages. However, Δ∈ should preferably be 260 or less and in particularnot higher than 200, as this would be detrimental for an at leastreasonably high specific resistivity, which in turn is anotherrequirement, especially for active matrix addressing. Most preferably Δ∈should be in the range of 50 to 180, more preferably either in the rangeof 60 to 90 or in the range from 100 to 160.

The displays according to the present invention are preferably activematrix LCDs, short AMDs, addressed by an active matrix, preferably by amatrix of thin film transistors (TFTs). However, the inventive liquidcrystals can also beneficially be used in displays with other knownaddressing means.

Liquid crystal compositions suitable for LCDs and in particular forTN-displays are already widely known. These compositions, however, dohave significant drawbacks. Most of them, besides having otherdeficiencies, lead to unfavourably high response times and/or tocontrast ratios, which are too low for many applications. They also mostgenerally have insufficient reliability and stability, in particularagainst exposure to heat, moisture or irradiation by light and inparticular UV, especially when one or more these stressors are combinedwith each other.

Thus, there is a significant need for liquid crystalline media withimproved suitable properties for practical applications such as a widenematic phase range, appropriate optical anisotropy Δn, according to thedisplay mode used, a high value of Δ∈, low viscosities, in particularlow rotational viscosities (γ₁), high contrast ratios in displays andespecially fast response times and a good reliability.

PRESENT INVENTION

Surprisingly, it now has been found that liquid crystalline media with asuitable phase range, suitably high values of Δ∈ and Δn and suitably lowviscosities can be realized, which allow stabilization with respectivepolymerisable compounds and, hence allow the realization of displays donot exhibit the drawbacks of the displays of the prior art or at leastdo exhibit them to a significantly lesser degree.

The present invention relates to liquid crystalline media, preferably todielectrically positive, chiral nematic media, comprising a chiralcomponent, component A, consisting of one or more chiral compounds,optionally, a bimesogenic component, component B, consisting of one ormore bimesogenic compounds, a liquid crystalline component, component C,consisting of one or more liquid crystalline, respectively mesogeniccompounds, a reactive component, component D, preferably a mesogeniccomponent, comprising, preferably consisting of, one or more reactivemesogenic compounds and optionally a polymerisation initiator, to amethod of their stabilisation by polymerising the reactive component andto liquid crystal displays comprising these media, respectively thesepolymer stabilised materials, especially to displays addressed by anactive matrix.

The improved liquid crystalline displays according to the instantapplication preferably fulfil the following conditions. They preferablycomprise

-   -   one or more, preferably one or two and, most preferably a pair        of substrates    -   at least one of them, preferably only one of them, bearing one        or more electrodes, preferably interdigital electrodes, more        preferably chevon type electrodes, and    -   at least one of them, preferably each one of them, bearing an        orientation layer for planar alignment of liquid crystalline        media or being otherwise treated for planar orientation of        liquid crystalline media,    -   a liquid crystal material according to the present invention.

The improved liquid crystalline mixtures according to the instantapplication preferably fulfil the following conditions. They preferably

-   -   either comprise        -   a chiral component, component A, consisting of one or more            chiral compounds,        -   optionally, a bimesogenic component, component B, consisting            of one or more bimesogenic compounds, preferably with a            symmetric structure and an odd number of atoms in the spacer            group between the two mesogenic units and/or one or more            bimesogenic compounds with a non-symmetric structure,            preferably with an odd number of atoms in the spacer group            between the two mesogenic units,        -   a liquid crystalline component, component C, consisting of            one or more liquid crystalline, respectively mesogenic            compounds, which preferably are non-reactive and            non-bimesogenic compounds, and        -   a reactive mesogenic component, component D, comprising,            preferably consisting of, one or more reactive mesogenic            compounds    -   or comprise        -   a chiral component, component A, consisting of one or more            chiral compounds,        -   optionally, a bimesogenic component, component B, consisting            of one or more bimesogenic compounds, preferably with a            symmetric structure and an odd number of atoms in the spacer            group between the two mesogenic units and/or one or more            bimesogenic compounds with a non-symmetric structure,            preferably with an odd number of atoms in the spacer group            between the two mesogenic units,        -   a liquid crystalline component, component C, consisting of            one or more liquid crystalline, respectively mesogenic            compounds, which preferably are non-reactive and            non-bimesogenic compounds,    -   and are stabilized by a polymer preparable from respective        precursors.

Preferably the cholesteric liquid crystal material according to thepresent invention is aligned in a uniformly standing helix structurewith a helical pitch preferably of 400 nm or less, more preferably of350 nm or less, and, most preferably of 320 nm or less.

Further, preferably the following conditions are fulfilled:

-   -   both substrates are bearing an orientation layer for planar        alignment of liquid crystalline materials or being otherwise        treated for planar alignment of liquid crystalline materials        and/or    -   the liquid crystalline material comprises, preferably consists        of        -   a liquid crystalline medium, having low molecular weight,            preferably exhibiting a nematic phase, respectively a            cholesteric phase, and        -   a polymeric material, preferably a mesogenic polymer, more            preferably a liquid crystalline polymer, and preferably        -   either the liquid crystalline medium is dispersed in the            polymer material or vice versa.

Preferably the respective components of cholesteric liquid crystalmaterial fulfil the following conditions:

-   -   the chiral component, component A,        -   consists of one or more chiral compounds, preferably of on            or more mesogenic compounds and/or        -   comprises one or more chiral compounds exhibiting an            absolute value of the HTP of 50 μm⁻¹ or more, preferably of            80 μm⁻¹ or more, and/or    -   the bimesogenic component, component B,        -   comprises one or more bimesogenic compounds, with a            symmetric structure, preferably with an odd number of atoms            in the spacer group between the two mesogenic units and/or        -   comprises one or more bimesogenic compounds with a            non-symmetric structure, preferably with an odd number of            atoms in the spacer group between the two mesogenic units            and/or    -   the liquid crystalline component, component C,        -   comprises one or more dielectrically positive compounds            and/or        -   comprises one or more dielectrically neutral and/or        -   consists of one or more liquid crystalline compounds,            respectively mesogenic compounds,        -   consists of non-reactive and non-bimesogenic compounds only,            and/or        -   exhibits a nematic phase, preferably over a range from 0° C.            or less to 80° C. or more, and/or        -   exhibits a dielectric anisotropy of 40 or more, preferably            of 50 or more and/or        -   exhibits a birefringence of 0.14 or more, preferably of 0.15            or more and/or    -   the reactive mesogenic component, component D        -   is capable to act as a polymer precursor and/or        -   comprises one or more mono-reactive mesogenic compounds            and/or        -   comprises one or more poly-reactive, preferably di-reactive,            mesogenic compounds and/or        -   optionally comprises one or more isotropic reactive            compounds and/or        -   optionally comprises one or more polymerisation initiators.

With regard to the chiral component, component A it is preferred thatsaid component is a mesogenic component, preferably consisting of chiralcompounds, preferably comprising one or more compounds selected from thegroup of formulae I, I′ and I″

wherein

-   R¹¹, R¹² and R¹³ are each, independently of one another, H, F, Cl,    CN, NO₂, NCS, SCN, OCN, a straight-chain or branched alkyl group    with 1 to 25 C atoms which may be unsubstituted, mono- or    polysubstituted by halogen or CN, it being also possible for one or    more non-adjacent CH₂ groups to be replaced, in each case    independently from one another, by —O—, —S—, —NH—, —N(CH₃)—, —CO—,    —COO—, —OCO—, —OCOO—, —S—CO—, —CO—S—, —CH═CH—, —CH═CF—, —CF═CH—,    —CF═CF— or —C≡C— in such a manner, that oxygen atoms are not linked    directly to one another, or, in case they are not linked to an O    atom;-   Y has the meaning given for R¹¹ and is preferably H, F, CH₃ or CF₃,    more preferably H or F;-   SP¹¹, SP¹² and SP¹³ are each, independently of one another, a    divalent spacer group comprising 1 to 40, preferably 4 to 20, C    atoms, preferably an alkylene group, it being also possible for one    or more CH₂ groups in the spacer groups to be replaced, in each case    independently from one another, by —O—, —S—, —NH—, —N(CH₃)—, —CO—,    —COO—, —OCO—, —OCOO—, —S—CO—, —CO—S—, —CH═CH—, —CH═CF—, —CF═CH—,    —CF═CF—, —CF₂— or —C≡C— in such a manner, that oxygen atoms are not    linked directly to one another;-   SP¹⁴ and SP¹⁵ are each, independently of one another, a divalent    spacer group comprising 1 to 40, preferably 4 to 20, C atoms;-   X¹¹, X¹² and X¹³ are each, independently of one another, —O—, —S—,    —CO—, —COO—, —OCO—, —OCOO—, —CO—NH—, —NH—CO—, —CH₂CH₂—, —OCH₂—,    —CH₂O—, —SCH²—, —CH₂S—, —CF═CF—, —CH═CH—, —OCO—CH═CH—, —C≡C— or a    single bond;-   k, l, n, m, p and q each are each, independently of one another, 0    or 1, with m being preferably 1;-   m+n+q is 1, 2 or 3, for formula I preferably 2, for formula I′    preferably 1 and for formula I″ preferably 2 or 3 and most    preferably 2;-   MG¹¹, MG¹² and MG¹³ are each, independently of one another, a    mesogenic group, preferably of the formula I′″    -A¹¹-(Z¹-A¹²)_(i1)-  I′″    wherein    -   A¹¹ and A¹² are each, independently of one another, a bivalent        ring group containing preferably at least four C atoms,        preferably a five- or a six-membered ring and preferably has the        meaning given for ring A³¹ under formula III below;    -   Z¹ each have, independently of one another, the meaning given        for Z³¹ under formula III below; and    -   i1 is 0, 1 or 2, preferably 0 or 1;-   CH*¹¹ is a chiral, bivalent group, preferably with a chiral center    or with one or more chiral atoms preferably selected from the group    of formulae Ia to In

-   -   or their mirror images, if not depicted;

-   CH*¹² is a chiral, bivalent group, preferably with a chiral center    or with one or more chiral atoms preferably selected from the group    of formulae Ie, If and In, or their mirror images, if not depicted,    especially preferred In; and

-   CH*¹³ is a chiral, trivalent group, preferably with a chiral center    or with one or more chiral atoms preferably —CH═, —CF═, —C(CH₃)═,    —C(OCH₃)═ or —C(CF₃)═;    where in all groups Ia to Ik, and especially preferred in the    aromatic rings of groups Ij, Ik, Il and In, optionally one or more    hydrogen atoms can be replaced by further aromatic rings, aliphatic    rings, alkyl chains, alkoxy chains, alkenyl chains and alkenyloxy    chains, which all may be substituted by halogen atoms, especially    fluorine, or CN.

With regard to the optional, bimesogenic component, component B, of theliquid crystal medium according to the invention said component ispreferably a mesogenic component preferably consisting of bimesogeniccompounds, preferably comprising one or more compounds of formula II,

wherein

-   R²¹ and, R²² are each, independently of one another, F, Cl, CN, NO₂,    NCS, SCN, OCN, a straight-chain or branched alkyl group with 1 to 25    C atoms which may be unsubstituted, mono- or polysubstituted by    halogen or CN, it being also possible for one or more non-adjacent    CH₂ groups to be replaced, in each case independently from one    another, by —O—, —S—, —NH—, —N(CH₃)—, —CO—, —COO—, —OCO—, —OCOO—,    —S—CO—, —CO—S—, —CH═CH—, —CH═CF—, —CF═CH—, —CF═CF— or —C≡C— in such    a manner, that in the whole molecule oxygen atoms are not linked    directly to one another, or, in case they are not linked to an O    atom, one or both of them may be H;-   MG²¹ and MG²² are each, independently of one another, a mesogenic    group and have preferably the meaning given for MG¹¹ under formula I    above,-   SP² is a divalent spacer group comprising 1 to 40, preferably 3 to    20, C atoms, preferably an alkylene group, it being also possible    for one or more CH₂ groups in the spacer groups to be replaced, in    each case independently from one another, by —O—, —S—, —NH—,    —N(CH₃)—, —CO—, —COO—, —OCO—, —OCOO—, —S—CO—, —CO—S—, —CH═CH—,    —CH═CF—, —CF═CH—, —CF═CF—, —CF₂— or —C≡C— in such a manner, that    oxygen atoms are not linked directly to one another;-   i and j are, independently of each other, 0 or 1;    wherein the moiety R²¹—[—O—]_(I)-MG²¹- either is identical to the    moiety R²²—[—O—]_(j)-MG²², i.e. the compound of formula II is    symmetric, or the moiety R²¹—[—O—]_(I)-MG²¹- is different from the    moiety R²²—[—O—]_(j)-MG²²-, i.e. the compound of formula II is    non-symmetric.

Preferably the media comprise besides said at least one non-symmetricbimesogenic compound also one or more bimesogenic compounds having asymmetric structure.

With regard to the liquid crystalline component, component C, it ispreferred that said component is consisting of dielectrically positivecompounds and optionally of dielectrically neutral compounds and/ordielectrically negative compounds. It preferably comprises one or moredielectrically positive compounds of formula III and, optionally,further dielectrically positive compounds

wherein

-   R³ is H, F, alkyl, alkoxy, fluorinated alkyl or fluorinated alkoxy    with 1 to 7 C-atoms, alkenyl, alkenyloxy, alkoxyalkyl or fluorinated    alkenyl with 2 to 7 C-atoms;-   X³ CN or NCS;-   Z³¹ and Z³², independently of each other, and in case Z³¹ is present    twice, also these independently of each other, are —CH₂—CH₂—,    —CF₂—CF₂—, —CF₂—O—, —O—CF₂—CO—O— or a single bond, preferably    —CH₂—CH₂—, —CO—O— or a single bond;

independently of each other, and in case

is present twice, also these independently of each other, are

-   K is 0, 1 or 2, preferably 0 or 1.

Optionally the liquid crystalline component, component C, comprises oneor more dielectrically negative compounds, preferably selected from thegroup of compounds of formula IV

wherein

-   R⁴¹ and R⁴² independently of each other have the meaning given for    R³ under formula III above:

one of

-   -   and the other one has, or the other two, independently of each        other, have the same meaning, or one of the meanings given for

-   -   or are

-   -   optionally one of

-   -   preferably

-   -   is, or both are

-   L⁴¹ and L⁴² are, independently of each other, ═C(—F)— or ═N—,    preferably at least one of them is ═C(—F)— and, most preferably,    both of them are ═C(—F)—;-   Z⁴¹ and Z⁴² are, independently of each other, —CH₂CH₂—, —COO—,    trans-CH═CH—, trans-CF═CF—, —CH₂O—, —CF₂O— or a single bond,    preferably at least one of them is a single bond and most preferably    both are a single bond; and-   L is 0 or 1 or 2, preferably 0 or 1.

Optionally, preferably obligatorily, the liquid crystalline component,component C, comprises one or more dielectrically neutral compounds,preferably selected from the group of compounds of formula V

wherein

-   R⁵¹ and R⁵², independently of each other, have the meaning given for    R³ under formula III above;-   the rings A⁵¹, A⁵² and A⁵³,    -   independently of each other, and in case ring A⁵² is present        twice, also these independently of each other, have the meaning        given for ring A³¹ under formula III above;-   Z⁵¹ and Z⁵², independently of each other, have the meaning given for    Z³¹ under formula III above; and-   M is 0 or 1 or 2, preferably 0 or 1.

Preferably the liquid crystalline component, component C, additionallyor alternatively to the compounds of formula III, comprises one or moredielectrically positive compounds selected from the group of compoundsof formula VI

wherein

-   R⁶ has the meaning given for R³ under formula III above;-   the rings A⁶¹, A⁶² and A⁶³,    -   independently of each other have the meaning given for ring A³¹        under formula III above and preferably

-   -   are independently of each other

-   -   more preferably

-   X⁶ is F, Cl, —CF₃, —OCF₂H or —OCF₃, preferably F or —OCF₃;-   Y⁶¹ and Y⁶² are independently of one another H or F, preferably Y⁶¹    is F and Y⁶² is H or F;-   Z⁶ is —COO—, —CF₂O—, —CH₂CH₂—, —CH═CH— or a single bond, preferably    —COO—, —CF₂O— or a single bond, more preferably —COO— or a single    bond and, most preferably, a single bond; and-   N and O are independently of one another 0 or 1, the sum of-   (N+O) preferably is 1 or 2.

In a preferred embodiment of the present invention the liquidcrystalline media according to the instant application comprise one ormore polymerisable compounds. These polymerisable compounds may benon-mesogenic compounds, like e.g. the well known EHA, resp. 2EHA, ormesogenic compounds. These polymerisable mesogenic compounds are calledhere “reactive mesogens” (short RMs). These polymerisable compounds,whether mesogenic or non-mesogenic, may be mono-reactive ormulti-reactive, preferably di-reactive. Preferably the media compriseboth one or more mono-reactive compounds and one or more multi-reactive,preferably di-reactive compounds. Most preferably the media comprise oneor more RMs, while non-mesogenic compounds may be present additionally.

The RMs can be chiral or achiral, and can comprise anacrylate/methacrylate group or another polymerisable group. In anespecially preferred embodiment the RM are chiral compounds, as thisallows the simple adjustment of the wavelength of the selectivereflection by polymerising a certain amount of the chiral RM, which thusis no longer available to twist the liquid crystal material, leading toan increased cholesteric pitch and consequently to selective reflectionat longer wavelengths. The resultant cholesteric pitch may bebeneficially stabilized against further change e.g. by use of anappropriate filter (e.g. UV filter) protecting the liquid crystal fromambient radiation.

In case chiral reactive mesogens are used in the liquid crystallinemedia according to the present invention, in many cases it is desirableto use a photo initiator in the media, too, when an exposure to UVradiation is applied. The use of a photo initiator leads to asignificant reduction of the dose of UV radiation required.

The host mixture contains liquid crystalline compounds having a lowmolar mass and preferably an amount of one or more chiral dopantssufficient to lead to selective reflection at a wavelength outside of,and, preferably below, the visible range of the electromagneticspectrum. These cholesteric phases with a relatively short cholestericpitch preferably are stabilised by a polymer. The stabilisation of the(cholesteric) phase is carried out by adding to the chiral liquidcrystalline host mixture one or more polymerisable compounds, preferablyRMs, preferably a mixture comprising mono-reactive and di-reactive RMs,plus a suitable photo-initiator, and polymerising the polymerisablecompounds, for example by exposure to UV irradiation, for a short time.Preferably the polymerisation is carried out in electro-optical cellsmaintained at a temperature in the cholesteric phase of the chiralliquid crystalline host mixture.

In case the media comprise one or more polymerisable compounds theypreferably additionally comprise one or more polymerisation initiators,e.g. photo initiators and/or thermal initiators.

The liquid crystalline media according to the present invention may beand in a preferred embodiment are stabilized by polymerisation ofrespective polymer precursors are consisting of said one or morepolymerisable compounds and optionally one or more of said initiators.Preferably the stabilising polymer has the morphology of a polymernetwork, i.e. the liquid crystalline material having a low molecularweight, i.e. the non-polymerisable liquid crystalline material/mesogenicmaterial is present in a more or less continuous form interspersed withmore or less smoth strands of polymeric material. Polymer networkstabilised liquid crystals are disclosed e.g. in Dierking, I., Adv.Mater. 12, No. 3, pp. 167-181 (2000).

In a preferred embodiment of the present invention the liquidcrystalline media according to the instant application comprise one ormore polymerisable compounds and preferably RMs.

The mesogenic mono-reactive compounds used according to the presentinvention preferably comprise one or more ring elements, linked togetherby a direct bond or via a linking group and, where two of these ringelements optionally may be linked to each other, either directly or viaa linking group, which may be identical to or different from the linkinggroup mentioned. The ring elements are preferably selected from thegroup of four-, five-, six- or seven-, preferably of five- or six-,membered rings.

The RMs used according to the present invention are preferably selectedfrom the group of formulae VIIA and VIIBR⁷¹-MG⁷¹-X⁷¹—SP⁷¹—PG⁷¹  VIIAPG⁷²-SP⁷²—X⁷²-MG⁷²-X⁷³—SP⁷³—PG⁷³  VIIBwherein

-   R⁷¹ is H, F, Cl, Br, I, CN, NO₂, NCS, SF₅, SO₂CF₃ or alkyl which is    straight chain or branched, preferably has 1 to 20 C-atoms, is    unsubstituted, mono- or poly-substituted by F, Cl, Br, I or CN, and    in which one or more non-adjacent CH₂ groups are optionally    replaced, in each case independently from one another, by —O—, —S—,    —NH—, —NR⁰¹—, —SiR⁰¹R⁰²—, —CO—, —COO—, —OCO—, —OCO—O—, —S—CO—,    —CO—S—, —CY⁰¹═CY⁰² or —C≡C— in such a manner that O and/or S atoms    are not linked directly to one another, preferably H, Halogen,    n-alkyl, n-alkoxy with 1 to 7 C-atoms preferably 2 to 5 C-atoms,    alkenyl, alkenyloxy or alkoxyalkyl with 2 to 7 C-atoms, preferably    with 2 to 5 C-atoms or CN, NCS, halogen, preferably F, Cl,    halogenated alkyl, alkenyl or alkoxy, preferably mono-, di- or    oligo-fluorinated alkyl, alkenyl or alkoxy, especially preferred    CF₃, OCF₂H or OCF₃,-   R⁰¹ and R⁰² are, independently of each other, H or alkyl with 1 to    12 C-atoms,

-   MG⁷¹ is a mesogenic moiety, preferably comprising one or more rings    and most preferably is a divalent radical of the formula

-   -   are, independently of each other, an aromatic and/or alicyclic        ring, or a group comprising two or more fused aromatic or        alicyclic rings, wherein these rings optionally contain one or        more hetero atoms selected from N, O and/or S, and are        optionally mono- or poly-substituted by R⁷²,

-   Z⁷¹ to Z⁷⁴ are, independently of each other, —O—, —S—, —CO—, —CO—O—,    —O—CO—, —S—CO—, —CO—S—, —O—CO—O—, —CO—NR⁰¹—, —NR⁰¹—CO—, —OCH₂—,    —CH₂O—, —SCH₂—, —CH₂S—, —CF₂O—, —OCF₂—, —CF₂S—, —SCF—, —CH₂CH₂—,    —CF₂CH₂—, —CH₂CF₂—, —CF₂CF₂—, —CH═N—, —N═C H—, —N═N—, —CH═CR⁰¹—,    —CY⁰¹═CY⁰²—, —C≡C—, —(CH₂)₄—, —CH ═CH—CO—O—, —O—CO—CH═CH— or a    single bond,

-   Y⁰¹ and Y⁰² are, independently of each other, F, Cl or CN, and    alternatively one of them may be H,

-   R⁷² is H or alkyl, preferably H or alkyl with 1 to 10 C-atoms,

-   PG⁷¹ is a polymerisable or reactive group,

-   SP⁷¹ is a spacer group or single bond, and

-   X⁷¹ has one of the meanings given for Z⁷¹ and preferably is —O—,    —CO—O—, —O—CO—, —CF₂O—, —OCF₂—, —CH₂O—, —OCH₂— or a single bond,

-   MG⁷² has the meaning given for MG⁷¹ above,

-   PG⁷² and PG⁷³ independently of each other have one of the meanings    given for PG⁷¹ above,

-   SP⁷² and SP⁷³ independently of each other have one of the meanings    given for SP¹¹ above, and

-   X⁷² and X⁷³ independently of each other have one of the meanings    given for X⁷¹ above.

In a preferred embodiment of the present invention the precursor of thepolymer comprises, besides the compound(s) of formula VIIA one or moredi-reactive mesogenic monomers, preferably of formula VIIB.

The compounds of formulae VIIA and VIIB according to the presentinvention may be chiral compounds.

Preferably the chiral component, component A, comprises one or morecompounds selected from the group of formulae Ia, Ib and I′a, preferablyof formula I′ a

wherein the parameters have the respective meanings given under formulaI above and preferably

-   R¹¹ and R¹² are, independently of each other, alkyl or fluorinated    alkyl with 1 to 7 C-atoms or alkenyl, alkoxyalkyl or fluorinated    alkenyl with 2 to 7 C-atoms.

More preferably the chiral component, component A, comprises one or morecompounds selected from the group of formulae Ia-1, Ia-2, I′ a-1, Ia′-2and Ia′-3, preferably of formula I′ a-1,

wherein the parameters have the respective meanings given under formulaI above and preferably

-   R¹¹ and R¹² are, independently of each other, alkyl or fluorinated    alkyl with 1 to 7 C-atoms or alkenyl, alkoxyalkyl or fluorinated    alkenyl with 2 to 7 C-atoms.

In a preferred embodiment of the present invention, the bimesogeniccomponent, component B, comprises one or more compounds of formula II,wherein MG²¹ and MG²² are identical to each other.

In another preferred embodiment of the present invention the bimesogeniccomponent, component B, comprises one or more compounds of formula II,wherein MG²¹ and MG²² are different from each other.

Preferably the bimesogenic component, component B, comprises one or morecompounds of formula IIa, preferably one or more compounds each of atleast two different formulae selected from this group of formulae

wherein

-   R²¹ and R²², independently of each other have the meaning given    under formula II above;-   the rings A²¹¹, A²¹², A²²¹ and A²²², independently of each other    have the meaning given for ring A³¹ under formula III above;-   Z²¹ and Z²², independently of each other have the meaning given for    Z³¹ under formula III above;-   L²¹ and L²², independently of each other have the same meaning as    given for X¹¹ under formula I, I′ or I′ above;-   B² is CH₂, CFH, CF₂ or if present more than once any combination    thereof, preferably CH₂; and-   m is an integer from 1 to 19, preferably from 3 to 17 and most    preferably from 5 to 13; it is especially preferred that m is an odd    number.

Especially preferred are compounds of formulae IIa-1, IIa-2, IIa-3 andIIa-4:

wherein

-   X^(21a) and X^(22a), independently of each other are CN, F, Cl or    fluorinated alkyl or fluorinated alkoxy, each with 1 to 4 C-atoms,    preferably CN, CF₃, F or Cl, more preferably CN, F or Cl, most    preferably CN or F, whereby in formulae IIa-1 and IIa-2 X^(21a) and    X^(22a) preferably are different from each other;-   Z^(21a) and Z^(22a), independently of each other are —COO—, OCO—,    —O— or a single bond, preferably Z^(21a) and Z^(22a) are both —O—,    Z^(21a) is —CO—O— and Z^(22a) is —O—CO— or Z^(21a) is —O—CO— and    Z^(22a) is —CO—O—, and, most preferably, Z^(21a) and Z^(22a) are    both —O— or Z^(21a) is —O—CO— and Z^(22a) is −—CO—O—;-   L^(21a) and L^(21b) independently of each other are H or F; and-   n is an odd integer in the range from 1 to 17, preferably from 3 to    15, and, most preferably, from 5 to 13.

Preferably the liquid crystalline media according to the instantinvention comprise one or more bimesogenic compounds having an oddnumber of atoms in the spacer group, preferably one or more bimesogeniccompounds selected from the group of compounds of formulae IIa-1 toIIa-4, wherein n is an odd integer.

Optionally the bimesogenic component, component B comprises one or morecompounds selected from the group of formulae IIb-1 to IIb-4, which arealmost identical with the respective formulae IIa-1 to IIa-4 givenabove, wherein the parameters have the meanings given above, which,however are different in that X^(21a) and X^(22a) are identical informulae IIa-1 and IIa-2 and/or n is an even integer in the range from 2to 18.

In a preferred embodiment of the present invention the liquidcrystalline component, component C, of the liquid crystalline mediaaccording to the instant application comprises one or more compounds offormula III′

wherein

-   R³ is alkyl, alkoxy, fluorinated alkyl or fluorinated alkoxy,    alkenyl, alkenyloxy, alkoxyalkyl, fluorinated alkenyl or fluorinated    alkenyloxy, preferably alkyl or alkoxyalkyl and most preferably    n-alkyl,

-   Y³¹ and Y³² are independently of each other H or F, preferably Y³¹    is F and Y³² is H,-   Y³ is CN or NCS, preferably CN, and-   i is 0 or 1.

In a more preferred embodiment of the present invention the liquidcrystalline media according to the instant application comprise one ormore compounds of formula III′ selected from the compounds of itsfollowing sub-formulae III′-1 to III′-5, preferably selected fromformulae III′-2, III′-4 and III′-5, most preferably of formula III′-2and/or III′-5

wherein R³ has the respective meanings given under formula I above andpreferably is alkyl or alkenyl, most preferably n-alkyl or alk-3-enyl,and X³ preferably is CN.

Additionally or alternatively to one or more compounds selected from thegroup of formulae III′-1 to III′-5, the present invention the liquidcrystalline media according to the instant application may comprise oneor more compounds of formula III selected from the compounds of itsfollowing sub-formulae III′-6 to III′-8, preferably of formula III′-8

wherein R³ has the respective meanings given under formula I above andpreferably is alkyl or alkenyl, most preferably n-alkyl or alk-3-enyl,and X³ preferably is CN.

The liquid crystal mixtures according to the present inventionoptionally comprise one or more dielectrically negative compounds havinga dielectrical anisotropy of −1.5 or less, preferably in the range from−1.5 or less to −8 or more. In this case, the liquid crystal mixturesaccording to the present invention preferably comprise one or moredielectrically negative compounds selected from the following group offormulae IV-1 to IV-8

whereinR⁴¹ and R⁴² have the respective meaning given under formula IV above.

The liquid crystal mixtures according to the present inventionpreferably comprise one or more dielectrically neutral compounds. Thesecompounds have a dielectrical anisotropy in the range from −1.5 to +3.0.Preferably these compounds are selected from the following group offormulae V-1 to V-14, preferably of formula V-14

wherein

R⁵¹ and R⁵² have the respective meaning given under formula V above.

Preferably the liquid crystalline component, component C, comprises oneor more compounds selected from the following group of formulae VI-1 toVI-5, preferably one or more compounds each of at least two differentformulae selected from this group of formulae, most preferably selectedfrom the group of formulae VI-1, VI-2, VI-3 and VI-5,

wherein

-   R⁶ has the meaning given under formula VI above. X³ is F, Cl or    fluorinated alkyl or fluorinated alkoxy, each with 1 to 4 C-atoms,    preferably OCF₃, OCF₂H, F or Cl, most preferably F.

Particularly preferred are polymer precursors comprising one or morecompounds of formula VIIA and/or of formula VIIB, wherein

-   -   Z⁷¹ and/or Z⁷⁴ is —O—, —CO—O—, —OCO—, —O—CO—O—, —CH₂—O—,        —O—CH₂—, —CF₂—O—, —O—CF₂—, —C≡C—, —CH═CH— or a single bond, most        preferably —CO—O— or —O—CO— or —O— and/or    -   Z⁷¹ is different from a single bond and/or    -   ring A⁷¹ is phenylene that is optionally substituted by one or        more groups R and/or    -   R⁷¹ is alkyl or alkoxy with 1 to 12, preferably 1 to 8 C-atoms,        or alkenyl, alkenyloxy or alkynyl with 2 to 12, preferably 2 to        7 C-atoms and/or    -   SP⁷¹ is alkylene with 1 to 12 C atoms which is optionally mono-        or polysubstituted by F and wherein one or more non-adjacent CH₂        may be replaced, in each case independently from one another, by        —O—, —CH═CH— or —C≡C—, and that is linked to a ring, preferably        to ring “A⁷¹” via a group selected from —O—, —CO—O—, —O—CO—,        —O—CO—O— and a single bond and/or    -   SP⁷¹ is a single bond.

Preferences for MG⁷² to X⁷³ are the same as for the respectivecorresponding groups MG⁷¹ to X⁷¹.

In a preferred embodiment rings A⁷¹ to A⁷³ are, independently of eachother, an aromatic or alicyclic ring, preferably a 5-, 6- or 7-memberedring, or a group comprising two or more, preferably two or three, fusedaromatic or alicyclic rings, wherein these rings optionally contain oneor more hetero atoms selected from N, O and/or S, and are optionallymono- or poly-substituted with L⁷, wherein L⁷ is F, Cl, Br, CN, OH, NO₂,and/or an alkyl, alkoxy, alkylcarbonyl or alkoxycarbonyl group with 1 to12 C atoms, wherein one or more H atoms are optionally replaced by F orCl.

L⁷ is preferably F, Cl, CN, OH, NO₂, CH₃, C₂H₅, OCH₃, OC₂H₅, COCH₃,COC₂H₅, COOCH₃, COOC₂H₅, CF₃, OCF₃, OCHF₂ or OC₂F₅, in particular F, Cl,CN, CH₃, C₂H₅, OCH₃, COCH₃ or OCF₃, most preferably F, Cl, CH₃, OCH₃ orCOCH₃.

Preferred rings A⁷¹ to A⁷³ are, for example, furan, pyrrol, thiophene,oxazole, thiazole, thiadiazole, imidazole, phenylene, cyclohexylene,cyclohexenylene, pyridine, pyrimidine, pyrazine, azulene, indane,naphthalene, tetrahydronaphthalene, decahydronaphthalene,tetrahydropyrane, anthracene, phenanthrene and fluorene.

Particularly preferably one or more of these rings A⁷¹ to A⁷³ isselected from furane-2,5-diyl, thiophene-2,5-diyl,thienothiophene-2,5-diyl, dithienothiophene-2,6-diyl, pyrrol-2,5-diyl,1,4-phenylene, azulene-2,6-diyl, pyridine-2,5-diyl, pyrimidine-2,5-diyl,naphthalene-2,6-diyl, 1,2,3,4-tetrahydro-naphthalene-2,6-diyl,indane-2,5-diyl, or 1,4-cyclohexylene wherein one or two non-adjacentCH₂ groups are optionally replaced by O and/or S, wherein these groupsare unsubstituted, mono- or polysubstituted by L as defined above.

Preferably

-   -   independently of each other are,

-   -   or their mirror images        wherein

-   R is alkyl with 1 to 12 C-atoms, preferably with 1 to 7 C-atoms, or    alkenyl or alkynyl with 2 to 12 C-atoms, preferably with 2 to 7    C-atoms, in both of which one or more non-adjacent —CH₂— groups, not    adjacent to the phenyl ring, may be replaced by —O— and/or —CH═CH—    and/or one or more H-atoms may be replaced by halogen, preferably by    F,    and/or preferably

In a preferred embodiment of the present invention the group

contains only monocyclic rings A⁷¹ to A⁷³. Very preferably this is agroup with one or two 5- and/or 6-membered rings.

Preferred sub formulae for this group are listed below. For reasons ofsimplicity, Phe in these groups is 1,4-phenylene, PheL is a1,4-phenylene group which is substituted by 1 to 4 groups L as definedabove, Cyc is 1,4-cyclohexylene, Pyd is pyridine-2,5-diyl and Pyr ispyrimidine-2,5-diyl. The following list of preferred groups comprisesthe sub formulae VII-1 to VII-20 as well as their mirror images,-Phe-  VII-1-Pyd-  VII-2-Pyr-  VII-3-PheL-  VII-4-Cyc-  VII-5-Phe-Z-Cyc-  VII-6-Cyc-Z-Cyc-  VII-7-PheL-Cyc-  VII-8-Phe-Z-Phe-  VII-9-Phe-Z-Pyd-  VII-10-Pyd-Z-Phe-  VII-11-Phe-Z-Pyr-  VII-12-Pyr-Z-Phe-  VII-13-PheL-Z-Phe-  VII-14-PheL-Z-Pyd-  VII-15-PheL-Z-Pyr-  VII-16-Pyr-Z-Pyd-  VII-17-Pyd-Z-Pyd-  VII-18-Pyr-Z-Pyr-  VII-19-PheL-Z-PheL-  VII-20

In these preferred groups Z has the meaning of Z⁷¹ as given in formulaVIIA. Preferably Z is —COO—, —OCO—, —CH₂CH₂—, —C≡C— or a single bond.Very preferably the group

is selected from the following formulae VIIIa to VIIj and their mirrorimages

wherein L is F, Cl, Br, CN, OH, NO₂, and/or an alkyl, alkoxy,alkylcarbonyl or alkoxycarbonyl group with 1 to 12 C atoms, wherein oneor more H atoms are optionally replaced by F or Cl and r is 0, 1, 2, 3or 4, preferably 0, 1 or 2.

in these preferred formulae is very preferably

furthermore

with L having each independently one of the meanings given above.

Especially preferred compounds of formula I comprise at least one group

wherein r is 1 or 2

Further preferred compounds of formula I comprise at least two groups

wherein r is 1 and/or at least one group

wherein r is 2.

-   -   more preferably is

-   -   wherein the 1,4-phenylene rings may optionally be substituted by        R, preferably by alkyl, preferably by methyl, and/or by alkoxy        and/or by halogen, preferably F.

More preferably

-   -   or also their respective mirror images, where not explicitly        given,        wherein R has the meaning given above and preferably is alkyl,        preferably with 1 to 6 C-atoms, preferably n-alkyl, wherein one        or more non-adjacent —CH₂— groups optionally may be replaced by        —O— and/or by —CH═CH— and/or one or more H-atoms may be replaced        by halogen, preferably by F.

Preferably the liquid crystalline media according to the instantinvention comprise, more preferably predominantly consist of, morepreferably essentially consist of and most preferably entirely consistof compounds selected from the group of compounds of formulae I to VIand VIIa and VIIb, more preferably of formulae I, II, III, V, VI andVIIa and/or VIIb.

“Comprising” in this application means in the context of compositionsthat the entity referred to, e.g. the medium or the component, containsthe component or components or of the compound or compounds in question,preferably in a total concentration of 10% or more and most preferablyof 20% or more unless explicitly defined otherwise.

In this context the term “predominantly consisting of” means that theentity referred to contains 55% or more, preferably 60% or more and mostpreferably 70% or more of the component or components or of the compoundor compounds in question unless explicitly defined otherwise.

In this context the term “essentially consisting of” means that theentity referred to contains 80% or more, preferably 90% or more and mostpreferably 95% or more of the component or components or of the compoundor compounds in question unless explicitly defined otherwise.

In this context the term “entirely consisting of” means that the entityreferred to contains 98% or more, preferably 99% or more and mostpreferably 100.0% of the component or components or of the compound orcompounds in question unless explicitly defined otherwise.

Also other mesogenic compounds, which are not explicitly mentionedabove, can optionally and beneficially be used in the media according tothe instant invention. Such compounds are known to the expert in thefield.

The liquid crystal media according to the instant invention arecharacterised by a clearing point of 75° C. or more, preferably of 80°C. or more.

The Δ∈, at 589 nm (Na^(D)) and 20° C., of the liquid crystal mediaaccording to the instant invention preferably is in the range of 0.120or more to 0.200 or less, more preferably in the range of 0.130 or moreto 0.180 or less and most preferably in the range of 0.140 or more to0.160 or less.

The Δ∈, at 1 kHz and 20° C., of the liquid crystal medium according tothe invention preferably is 30 or more, preferably 40 or more, morepreferably 60 or more and most preferably 60 or more, whereas itpreferably is 100 or less, more preferably 80 or less and morepreferably it is in the range of 30 or more, to 90 or less and most inthe range of 40 to 80 and, most preferably in the range of 60 to 70.

Preferably the nematic phase of the inventive media without the chiraldopants extends at least from 0° C. or less to 75° C. or more, morepreferably at least from −20° C. or less to 75° C. or more, mostpreferably at least from −20° C. or less to 80° C. or more and inparticular at least from −30° C. or less to 80° C. or more.

The liquid crystalline media are preferably characterized for comparisonpurposes in TN displays operating in the second transmission minimumaccording to Gooch and Tarry having an optical retardation (d·Δn) in therange of 1.0 μm or more to 1.1 μm or less. They are, however, preferablyused as cholesteric liquid crystals, also called chiral nematic liquidcrystals, having a rather short cholesteric pitch, preferably theircholesteric pitch is selected such, that their wavelength of selectivereflection is in the in the range in the visible range of theelectromagnetic spectrum i.e. in the range from of 400 nm to 800 nm.

Preferably the liquid crystal media contain one or more chiral dopantspreferably having an absolute value of the helical twisting power (HTP)of 50 μm⁻¹ or more, preferably of 60 μm⁻¹ or more, more preferably inthe range of 70 μm⁻¹ or more, most preferably in the range of 80 μm⁻¹ ormore to 260 μm⁻¹ or less.

Preferably the liquid crystal media contain 50% to 100%, more preferably70% to 100% more preferably 80% to 100% and in particular 90% to 100%totally of compounds of formulae I, II, III, IV, V, VI and VIIA and/orVIIB, preferably of formulae I, II, III, V, VI, VIIA and VIIB.

More preferably the liquid crystal media comprise, more preferablypredominantly consist of, more preferably essentially consist of andmost preferably entirely consist of compounds of formulae I, II, III,IV, V, VI, VIIA and VIIB, preferably of formulae I, II, III, V, VI, VIIAand VIIB.

Compounds of formula I preferably are used in the media in a totalconcentration from 1% to 15%, more preferably from 2% to 10%, morepreferably from 3% to 8% and most preferably from 4% to 7% of the totalmixture.

Compounds of formula II preferably are used in the media, if present atall, in a total concentration from 0% to 55%, more preferably from 5% to20% and most preferably from 6% to 15% of the total mixture.

Compounds of formula III preferably are used in the media in a totalconcentration from 45% to 75%, more preferably from 50% to 7040% andmost preferably from 55% to 65% of component C.

Compounds of formula IV preferably are used in the media in a totalconcentration from 0% to 35%, more preferably from 0% to 15% and mostpreferably from 5% to 10% of component C, if present at all.

Compounds of formula V preferably are used in the media in a totalconcentration from 0% to 30%, preferably from 50% to 25% and mostpreferably from 10% to 20% of component C.

Compounds of formula VI preferably are used in the media in a totalconcentration from 5% to 40%, preferably from 10% to 35% and mostpreferably from 15% to 30% of component C.

The polymerisable compounds, preferably of formulae VIIA and/or VIIB,preferably are used in the media in a total concentration from 3% to25%, more preferably from 5% to 20% and most preferably from 6% to 15%of the total mixture, prior to polymerisation thereof.

Preferably one or more polymerisation initiators, preferably one or morephoto initiators are used. The concentration of the initiators is from0.1% to 10%, more preferably from 0.2% to 5% and most preferably from0.5% to 2% of the total concentration of the polymerisable compounds.

Optionally the media according to the present invention may comprisefurther liquid crystal compounds in order to adjust the physicalproperties. Such compounds are known to the expert. Their concentrationin the media according to the instant invention is preferably 0% to 30%,more preferably 0.1% to 20% and most preferably 1% to 15%.

Preferably the media according to the present invention comprise

-   -   one or more compounds of formula I and/or formula I′, preferably        of formula I′, more preferably of formula I′ a-1, most        preferably of formula R-5011 or S-5011, and/or    -   optionally one or more compounds of formula II, preferably of        formula IIa-2 and/or IIa-3, and/or    -   one or more compounds of formula III′, preferably of formulae        III′-2 and/or III′-5, and/or    -   one or more compounds of formula IV, preferably of formula V-13,        and/or    -   one or more compounds of formula VI, preferably of formulae V-1,        and/or VI-2 and/or VI-3 and/or VI-5, and/or    -   one or more reactive polymerisable compounds, preferably one or        more compounds of formulae VIIA and/or VIIB-7, preferably both        one or more compounds of formula formulae VIIA and one or more        compounds of formula VIIB-7 and/or    -   one or more polymerisation initiators.

In the present application the term dielectrically positive is used forcompounds or components with Δ∈>3.0, dielectrically neutral with−1.5≦Δ∈≦3.0 and dielectrically negative with Δ∈<−1.5. Δ∈ is determinedat a frequency of 1 kHz and at 20° C. The dielectric anisotropy of therespective compound is determined from the results of a solution of 10of the respective individual compound in a nematic host mixture. In casethe solubility of the respective compound in the host mixture is lessthan 10% its concentration is reduced by a factor of 2 until theresultant mixture is stable enough at least to allow the determinationof its properties. Preferably the concentration is kept at least at 5%,however, in order to keep the significance of the results a high aspossible. The capacities of the test mixtures are determined both in acell with homeotropic and with homogeneous alignment. The cell gap ofboth types of cells is approximately 20 μm. The voltage applied is arectangular wave with a frequency of 1 kHz and a root mean square valuetypically of 0.5 V to 1.0 V, however, it is always selected to be belowthe capacitive threshold of the respective test mixture.

Δ∈ is defined as (∈_(∥)−∈_(⊥)), whereas ∈_(av) is (∈_(∥)+2 ∈_(⊥))/3. Fordielectrically positive compounds the mixture ZLI-4792 and fordielectrically neutral, as well as for dielectrically negativecompounds, the mixture ZLI-3086, both of Merck KGaA, Germany are used ashost mixture, respectively. The dielectric permittivities of thecompounds are determined from the change of the respective values of thehost mixture upon addition of the compounds of interest. The values areextrapolated to a concentration of the compounds of interest of 100%.

Components having a nematic phase at the measurement temperature of 20°C. are measured as such, all others are treated like compounds.

The term threshold voltage refers in the instant application to theoptical threshold and is given for 10% relative contrast (V₁₀) and theterm saturation voltage refers to the optical saturation and is givenfor 90% relative contrast (V₉₀) both, if not explicitly statedotherwise. The capacitive threshold voltage (V₀), also calledFreedericks-threshold (V_(Fr)) is only used if explicitly mentioned.

The ranges of parameters given in this application are all including thelimiting values, unless explicitly stated otherwise.

Throughout this application, unless explicitly stated otherwise, allconcentrations are given in mass percent and relate to the respectivecomplete mixture, all temperatures are given in degrees centigrade(Celsius) and all differences of temperatures in degrees centigrade. Allphysical properties have been and are determined according to “MerckLiquid Crystals, Physical Properties of Liquid Crystals”, StatusNovember 1997, Merck KGaA, Germany and are given for a temperature of20° C., unless explicitly stated otherwise. The optical anisotropy (Δn)is determined at a wavelength of 589.3 nm. The dielectric anisotropy(Δ∈) is determined at a frequency of 1 kHz. The threshold voltages, aswell as all other electro-optical properties have been determined withtest cells prepared at Merck KGaA, Germany. The test cells for thedetermination of Δ∈ had a cell gap of approximately 20 μm. The electrodewas a circular ITO electrode with an area of 1.13 cm² and a guard ring.The orientation layers were lecithin for homeotropic orientation (∈_(∥))and polyimide AL-1054 from Japan Synthetic Rubber for planar homogeneousorientation (∈_(⊥)). The capacities were determined with a frequencyresponse analyser Solatron 1260 using a sine wave with a voltage of 0.3V_(rms). The test cells used for the electro-optical measurements havecell gap selected to have an optical retardation in the range from 0.310μm⁻¹ to 0.32 μm⁻¹. They have interdigital electrodes, i.e. electrodes ofthe type used in IPS— displays. Alternatively also cells with chevrontype electrodes may be used. The light used in the electro-opticalmeasurements was white light. The set up used was commercially availableequipment of Autronic Melchers, Karlsruhe, Germany. The characteristicvoltages have been determined under perpendicular observation. Thethreshold (V₁₀)—mid grey (V₅₀)—and saturation (V₉₀) voltages have beendetermined for 10%, 50% and 90% relative contrast, respectively.

The response times are given as rise time (τ_(on)) for the time for thechange of the relative contrast from 0% to 90% (t₉₀−t₀), i.e. includingthe delay time (t₁₀−t₀), as decay time (τ_(off)) for the time for thechange of the relative contrast from 100% back to 10% (t₁₀₀−t₁₀) and asthe total response time (τ_(total))=τ_(on)+τ_(off)), respectively.

The liquid crystal media according to the present invention may containfurther additives in usual concentrations. The total concentration ofthese further constituents is in the range of 0% to 10%, preferably 0.1%to 6%, based on the total mixture. The concentrations of the individualcompounds used each are preferably in the range of 0.1% to 3%. Theconcentration of these and of similar additives is not taken intoconsideration for the values and ranges of the concentrations of theliquid crystal components and compounds of the liquid crystal media inthis application. This also holds for the concentration of the dichroicdyes used in the mixtures, which are not counted when the concentrationsof the compounds respectively the components of the host mixture arespecified. The concentration of the respective additives is always givenrelative to the final doped mixture.

The liquid crystal media according to the present invention consist ofseveral compounds, preferably of 3 to 30, more preferably of 4 to 20 andmost preferably of 4 to 16 compounds. These compounds are mixed inconventional way. As a rule, the required amount of the compound used inthe smaller amount is dissolved in the compound used in the greateramount. In case the temperature is above the clearing point of thecompound used in the higher concentration, it is particularly easy toobserve completion of the process of dissolution. It is, however, alsopossible to prepare the media by other conventional ways, e.g. using socalled pre-mixtures, which can be e.g. homologous or eutectic mixturesof compounds or using so called multi-bottle-systems, the constituentsof which are ready to use mixtures themselves.

Preferably the liquid crystal media according to the present invention,comprising one or more chiral dopants, is selectively reflectingradiation in a range outside of the visible range of the electromagneticspectrum, i.e. not in the range from 400 nm to 800 nm. Preferably theirband of selective reflection does not extend into this range ofwavelengths more preferably at least the centre wavelength of theirreflection band lies outside of this range and most preferably theircomplete reflection band lies outside of this range.

The wavelength of the centre of the resultant selective reflection at agiven temperature may be calculated from the actual concentration of thechiral dopant in the host used via the approximation of the polynomialseries (I):λ_(cent.) [c(dop.)]=α·[c(dop.)]⁻¹ +β·[c(dop.)]⁻² +γ·[c(dop.)]⁻³  (I)wherein

-   α, β and γ are material constants specific for the combination of a    given chiral dopant in a given host mixture and-   c(dop.) is the concentration of the chiral dopant in the host    mixture.

In many practical cases, even consideration only of the first term, thelinear term (“α·[c(dop.)]⁻¹”), yields results with sufficient accuracy.The parameter “a” is analogous to the inverse of the HTP (i.e. HTP⁻¹).Here, in the determination of the wavelength of the selective reflectionof a cholesteric LC, which is similar to a “Bagg” reflection, however,the effective refractive index of the mixture has to be taken intoaccount additionally for a more exact numerical description.

Typically the parameters α, β and γ do depend more strongly on the typeof the chiral dopant, than on the specific liquid crystal mixture used.

Obviously, they depend on the enantiomeric excess of the respectivechiral dopant. They have their respective largest absolute values arefor the pure enantiomers and are zero for racemates. In this applicationthe values given are those for the pure enantiomers, having anenantiomeric excess of 98% or more.

Preferably the absolute value of the parameter α of the chiral dopant,respectively the chiral dopants, in the respective liquid crystal mediumaccording to the present application is in the range from 5 nm to 25 nm,more preferably in the range from 10 nm to 20 nm and most preferably inthe range from 12 nm to 16 nm.

These media may comprise more than one chiral dopant. In case theycomprise two or more chiral dopants, these may beneficially selected inone of the known ways to compensate e.g. against the temperaturedependence of the cholesteric pitch and, hence, of the wavelength ofselective reflection. Here in one host mixture chiral dopants having thesame sign of the parameter α may be used as well as chiral dopantshaving the opposite sign of this parameter, depending on the nature ofthe parameters for the terms of higher order of equation (I), inparticular of the parameter β, the parameter of the quadratic term.

More preferred is an embodiment of the present invention using a singlechiral dopant, which shows a small temperature dependence of the chiralpitch induced in the respective host mixture, i.e. has a small parameterβ. By addition of suitable additives, the liquid crystal media accordingto the instant invention can be modified in such a way, that they areusable in all known types of liquid crystal displays, either using theliquid crystal media as such, like TN-, TN-AMD, ECB-AMD, VAN-AMD, IPSand OCB LCDs and in particular in composite systems, like PDLC, NCAP, PNLCDs and especially in ASM-PA LCDs.

The melting point T(C,N), the transition from the smectic (S) to thenematic (N) phase T(S,N) and the clearing point T(N,I) of the liquidcrystals are given in degrees centigrade.

In the present application and especially in the following examples, thestructures of the liquid crystal compounds are represented byabbreviations, which are also called “acronyms”. The transformation ofthe abbreviations into the corresponding structures is straight forwardaccording to the following three tables A to C.

All groups C_(n)H_(2n+1)) C_(m)H_(2m+1) and C_(l)H_(2l+1) are preferablystraight chain alkyl groups with n, m and I C-atoms, respectively, allgroups C_(n)H_(2n), C_(m)H_(2m) and C_(l)H_(2l) are preferably(CH₂)_(n), (CH₂)_(m) and (CH₂)_(m), respectively and —CH═CH— preferablyis trans-respectively E vinylene.

Table A lists the symbols used for the ring elements, table B those forthe linking groups and table C those for the symbols for the left handand the right hand end groups of the molecules.

Table D lists exemplary molecular structures together with theirrespective codes.

TABLE A Ring Elements C

P

D

DI

A

AI

G

GI

U

UI

Y

M

MI

N

NI

np

n3f

n3fI

th

thI

th2f

th2fI

o2f

o2fI

dh

K

KI

L

LI

F

FI

TABLE B Linking Groups E —CH₂—CH₂— V —CH═CH— T —C≡C— W —CF₂—CF₂— B—CF═CF— Z —CO—O— ZI —O—CO— X —CF═CH— XI —CH═CF— O —CH₂—O— OI —O—CH₂— Q—CF₂—O— QI —O—CF₂—

TABLE C End Groups Left hand side, used alone or in Right hand side,used alone or in combination with others combination with others -n-C_(n)H_(2n+1)— -n —C_(n)H_(2n+1) -nO- C_(n)H_(2n+1)—O— -nO—O—C_(n)H_(2n+1) -V- CH₂═CH— -V —CH═CH₂ -nV- C_(n)H_(2n+1)—CH═CH— -nV—C_(n)H_(2n)—CH═CH₂ -Vn- CH₂═CH—C_(n)H_(2n)— -Vn —CH═CH—C_(n)H_(2n+1)-nVm- C_(n)H_(2n+1)—CH═CH—C_(m)H_(2m)— -nVm—C_(n)H_(2n)—CH═CH—C_(m)H_(2m+1) -N- N≡C— -N —C≡N -S- S═C═N— -S —N═C═S-F- F— -F —F -CL- Cl— -CL —Cl -M- CFH₂— -M —CFH₂ -D- CF₂H— -D —CF₂H -T-CF₃— -T —CF₃ -MO- CFH₂O— -OM —OCFH₂ -DO- CF₂HO— -OD —OCF₂H -TO- CF₃O—-OT —OCF₃ -A- H—C≡C— -A —C≡C—H -nA- C_(n)H_(2n+1)—C≡C— -An—C≡C—C_(n)H_(2n+1) -NA- N≡C—C≡C— -AN —C≡C—C≡N Left hand side, used incombination Right hand side, used in with others only combination withothers only -...n...- —C_(n)H_(2n)— -...n... —C_(n)H_(2n)— -...M...-—CFH— -...M... —CFH— -...D...- —CF₂— -...D... —CF₂— -...V...- —CH═CH—-...V... —CH═CH— -...Z...- —CO—O— -...Z... —CO—O— -...ZI...- —O—CO—-...ZI... —O—CO— -...K...- —CO— -...K... —CO— -...W...- —CF═CF— -...W...—CF═CF—wherein n and m each are integers and three points “ . . . ” indicate aspace for other symbols of this table.

Preferably the liquid crystalline media according to the presentinvention comprise, besides the compound(s) of formula I one or morecompounds selected from the group of compounds of the formulae of thefollowing table.

TABLE D

In a similar manner the bimesogenic compounds are labelled. Here, firstthe central spacer group is given by the number n of —CH₂— followed bythe code for the respective mesogenic groups written in brackets.Preferably the liquid crystalline media according to the presentinvention comprise, besides the compound(s) of formula I one or morecompounds selected from the group of compounds of the formulae of thefollowing table.

TABLE E

Table F lists chiral dopants, which are preferably used in the liquidcrystalline media according to the present invention.

TABLE F

In a preferred embodiment of the present invention the media accordingto the present invention comprise one or more compounds selected fromthe group of compounds of table F.

Table G lists stabilizers, which are preferably used in the liquidcrystalline media according to the present invention.

TABLE G

Remark: In this table “n” means an integer in the range from 1 to 12.

In a preferred embodiment of the present invention the media accordingto the present invention comprise one or more compounds selected fromthe group of compounds of table F.

The liquid crystalline media according to the present invention comprisepreferably

-   -   four or more, preferably six or more, compounds selected from        the group of compounds of table D, preferably    -   seven or more, preferably eight or more compounds, preferably        compounds of three or more different formulae, selected from the        group of formulae of table D.

EXAMPLES

The examples given in the following are illustrating the presentinvention without limiting it in any way.

However, the physical properties and compositions illustrate for theexpert, which properties can be achieved and in which ranges they can bemodified. Especially the combination of the various properties, whichcan be preferably achieved, is thus well defined for the expert.

Liquid crystal mixtures are realized with the compositions andproperties given in the following tables. Their optical performance isinvestigated.

Comparative Example 0

The following liquid crystalline mixture consisting of bimesogens isprepared and investigated.

TABLE 1 Composition and Properties of Liquid Crystal Mixture A-0Composition Compound No. Abbreviation Conc./% 1 9(O-GP-F)₂ 25.0 29(O-GP-F) (O-PP-N) 25.0 3 9(Z-GP-F)₂ 25.0 4 9(Z-GP-F) (Z-PP-N) 25.0 Σ100.0 Physical Properties T (N, I) = n.d. ° C. Remarks: n.d.: notdetermined.

5% of the chiral dopant R-5011 available from Merck KGaA, Darmstadt,Germany, are added to 95.0% of the mixture A-0 from example 1 to preparea respective cholesteric mixtures called mixture A-1, having arelatively short cholesteric pitch. After addition of the chiral dopantthe mixture is heated to a temperature of 110° C. and kept at thistemperature for 10 minutes. Then it is allowed to cool down to ambienttemperature again.

TABLE 2a Comparison of Composition Example C. E. 0 C. E. 1 C. E. 2Mixture A-1 B-1 C-1 Material Concentration/% Composition A-0 95.0 0.00.0 B-0 0.0 95.0 0.0 0.0 0.0 95.0 R-5011 5.0 5.0 5.0 Σ 100.0 100.0 100.0PPhysical Properties T (N, I)/° C. n.d. n.d. n.d. Remarks: n.d.: notdetermined.

The mixture A-1 is filled into a LC cell of the IPS type having a cellgap in the range from 5.0 μm to 5.5 μm and having parallel stripeelectrodes with a separation between the adjacent electrodes of 9 μm.The inner surfaces of the substrates forming the test cells are coveredwith an alignment layer (PI) and treated by rubbing to achieve a planarorientation of the liquid crystalline material, leading to the so calleduniformly standing helix alignment.

The liquid crystalline mixture in this cell shows electro-clinicswitching. This cell requires a relatively high operation voltage. Thethreshold voltage being at more than 120 V and the saturation voltageeven being at more than 200 V. Further the electro-opticalcharacteristic shows both a marked hysteresis and a significant residualtransmission in the dark state of up to as much as 30% of the brightstate.

TABLE 2b Comparison of Results Example C:E 0 C. E. 1 C. E. 2 Mixture A-1B-1 C-1 Electro-optical performance V₁₀(on)/V 115 55 n.d. V₅₀(on)/V 17570 n.d. V₉₀(on)/V >200 85 n.d. V₉₀(off)/V 150 90 n.d. V₅₀(off)/V 25 70n.d. V₁₀(off)/V n.d. n.d. n.d. Trans.(off)/% 12 10 n.d. Remarks: n.d.:not determined.

Comparative Example 2

Next a strongly dielectrically positive liquid crystalline mixture (B-0)with the composition shown in the following table is realized.

TABLE 3 Composition and Properties of Liquid Crystal Mixture B-0Composition Compound No. Abbreviation Conc./% 1 PZG-2-N 9.0 2 PZG-3-N9.0 3 PZG-4-N 12.0  4 PZG-5-N 12.0  5 PZU-V2-N 13.0  6 CCG-3-OT 2.0 7CCU-3-F 4.0 8 CCU-5-F 4.0 9 CDU-3-F 4.0 10  CDU-5-F 4.0 11  CCGU-3-F10.0 12  CPZG-3-N 4.0 13  CPZG-4-N 3.0 14  CCPC-3-3 4.0 15  CCPC-3-4 3.016  CCPC-3-5 3.0 Σ 100.0  Physical Properties T (N, I) = 82.0° C. n_(e)(20° C., 589.3 nm) = 1.6349 Δn (20° C., 589.3 nm) = 0.1461 ε_(||) (20°C., 1 kHz) = 79.1 Δε (20° C., 1 kHz) = 67.7

5% of the chiral dopant R-5011 available from Merck KGaA, Darmstadt,Germany, are added to 95.0% of the mixture B-0 from example 1 to preparea respective cholesteric mixtures called mixture B-1, having arelatively short cholesteric pitch. After addition of the chiral dopantthe mixture is heated to a temperature of 110° C. and kept at thistemperature for 10 minutes. Then it is allowed to cool down to ambienttemperature again.

The mixture B-1 is filled into a LC cell of the IPS type andinvestigated as described under comparative example above. The resultsare shown in table 2b.

Example 1 Examples 1.1 to 1.4

In this example the host mixture B-0 from comparative example 2 is dopedagain with the chiral dopant R-5011, like in that comparative example.But now it is subsequently stabilized by polymerization of a polymerprecursor consisting of both a mono-reactive mesogen and a di-reactivemesogen. As mono-reactive mesogen “MRM-A”

is used and as di-reactive mesogen “DRM-A”

is used here. Also a polymerization initiator Irgacure651 from Ciba,Switzerland, called “IRG-651”

is used here.

The concentrations of the components are varied as show in the followingtable.

TABLE 4a Composition and Properties of Liquid Crystal Mixtures B-2.1 toB-2.4 Example 1.1 1.2 1.3 1.4 Mixture B-2.1 B-2.2 B-2.3 B-2.4 MaterialConc./% Composition A-0 84.8 88.8 85.8 89.8 R-5011 5.0 5.0 4.0 4.0 MRM-A5.0 3.0 5.0 3.0 DRM-A 5.0 3.0 5.0 3.0 IRG-651 0.2 0.2 0.2 0.2 Σ 100.0100.0 100.0 100.0 Physical Properties T (N, I)/° C. n.d. n.d. n.d. n.d.Remarks: n.d.: not determined.

These samples are the further processed as follows. After addition ofthe chiral dopant, in the amount corresponding to its respectiveconcentration in the final mixture, the resultant intermediate mixtureis heated to a temperature of 110° C. and kept at this temperature for10 minutes. Then it is allowed to cool down to ambient temperatureagain. Then the two reactive mesogens are added in their respectiveconcentrations together with the polymerization initiator. The finalmixture is heated to a temperature of 60° C. and kept at thistemperature for another 10 minutes. Then it is allowed to cool down toambient temperature again.

After filling the mixtures into the respective cells, these cells areplaced in a hot stage, heated to a temperature of 80° C. andsubsequently allowed to cool down to ambient temperature again. Thislatter part of the process furthers the uniform alignment of thematerial. Finally the reactive mesogens of the polymer pecursor ispolymerized by exposure to UV radiation (wavelength 365 nm) with aradiation power of 5 mW for 15 minutes.

The cells with the cured polymer, i.e. the polymerized polymerprecursors, are then investigated for their electro-optical performance.The results are shown in table . . .

TABLE 4b Comparison of Results Example 1.1 1.2 1.3 1.4 Mixture A-2.1A-2.2 A-2.3 A-2.4 Electro-optical performance V₁₀(on)/V 95 80 75 52V₅₀(on)/V 155 120 120 90 V₉₀(on)/V 200 140 175 105 V₉₀(off)/V 170 110145 85 V₅₀(off)/V 125 95 100 75 V₁₀(off)/V 95 73 75 51 Transm.(off)/% <1<1 <1 <1

Example 1.5

In this example, additionally to the compounds R-5011, MRM-1, DRM-1 andIRG-651, 10% of the bimesogic compound 9(Z-GP-F)(Z-PP-N) are used in thehost mixture B-0 from comparative example 2.

Comparative Example 2

Next another strongly dielectrically positive liquid crystalline mixture(C-0) with the composition shown in the following table is realized.

TABLE 3 Composition and Properties of Liquid Crystal Mixture C-0Composition Compound No. Abbreviation Conc./% 1 PZG-2-N 9.0 2 PZG-3-N9.0 3 PZG-4-N 14.0  4 PZG-5-N 10.0  5 PZU-V2-N 16.0  6 CU-3-N 2.0 7CPZG-3-N 4.0 8 CCZU-2-F 4.0 9 CCZU-3-F 8.0 10  CCZU-5-F 4.0 11  CCPC-3-35.0 12  CCPC-3-4 5.0 13  CCPC-3-5 5.0 14  CGPC-3-5 3.0 15  CGPC-5-5 3.0Σ 100.0  Physical Properties T (N, I) = 81.0° C. n_(e) (20° C., 589.3nm) = 1.6421 Δn (20° C., 589.3 nm) = 0.1436 ε_(||) (20° C., 1 kHz) =81.9 Δε (20° C., 1 kHz) = 69.55% of the chiral dopant R-5011 available from Merck KGaA, Darmstadt,Germany, are added to 95.0% of the mixture C-0 from example 1 to preparea respective cholesteric mixtures called mixture C-1, having arelatively short cholesteric pitch. After addition of the chiral dopantthe mixture is heated to a temperature of 110° C. and kept at thistemperature for 10 minutes. Then it is allowed to cool down to ambienttemperature again.

The mixture C-1 is filled into a LC cell of the IPS type andinvestigated as described under comparative example above. The resultsare shown in table 2b.

Example 2

Analogously to example 1 the mixture comprising the host mixture C-0 andthe chiral dopant R-5011 is polymer-stabilized.

The invention claimed is:
 1. A liquid crystal medium which comprises: achiral component, component A, consisting of one or more chiralcompounds, in an amount such that the medium exhibits selectivereflection at a wavelength outside of the visible range of theelectromagnetic spectrum ranging from 400 nm to 800 nm, a bimesogeniccomponent, component B, consisting of one or more bimesogenic compounds,a liquid crystalline component, component C, consisting of one or moreliquid crystalline mesogenic compounds, and a reactive mesogeniccomponent, component D, comprising, one or more mono-reactive mesogeniccompounds and one or more poly-reactive mesogenic compounds.
 2. A liquidcrystal medium according to claim 1, which further comprises: apolymerisation initiator.
 3. A liquid crystal medium according to claim1, wherein the liquid crystalline mesogenic compounds of component Ccomprise: one or more compounds of formula III′

wherein R³ is alkyl, alkoxy, fluorinated alkyl or fluorinated alkoxy,alkenyl, alkenyloxy, alkoxyalkyl, fluorinated alkenyl or fluorinatedalkenyloxy, preferably alkyl or alkoxyalkyl and most preferably n-alkyl,

Y³¹ and Y³² are independently of each other H or F, preferably Y³¹ is Fand Y³² is H, Y³ is CN or NCS, preferably CN, and i is 0 or
 1. 4. Aliquid crystal device comprising a liquid crystal medium according toclaim
 1. 5. A process for stabilizing a liquid crystal medium accordingto claim 1, which comprises polymerizing the reactive mesogeniccomponent, component D.
 6. A polymer stabilized liquid crystal materialobtained by polymerizing a liquid crystal medium according to claim 1.7. A liquid crystal display, which comprises a liquid crystal mediumaccording to claim
 1. 8. A liquid crystal display according to claim 7,wherein the liquid crystal medium is aligned in the uniformly standinghelix mode.
 9. A liquid crystal display according to claim 7,characterised in that it is addressable by an active matrix.
 10. Aprocess for the preparation of a composite comprising a liquidcrystalline material and a polymeric material by polymerisation of apolymer precursor in a liquid crystalline medium according to claim 1.11. A process for the preparation of a liquid crystal display comprisingpolymerizing a polymer precursor in a liquid crystalline mediumaccording to claim
 1. 12. A process for the preparation of a liquidcrystal display comprising polymerizing one or more reactive mesogeniccompounds of component D in a liquid crystalline medium according toclaim
 1. 13. A liquid crystal medium according to claim 1, wherein thechiral component A is provided in an amount such that the mediumexhibits selective reflection at a wavelength below of the visible rangeof the electromagnetic spectrum ranging from 400 nm to 800 nm.
 14. Aliquid crystal medium according to claim 1, wherein the bimesogeniccompounds of component B comprise: one or more compounds of formula II:

wherein R²¹ and, R²² are each, independently of one another, F, Cl, CN,NO₂, NCS, SCN, OCN, a straight-chain or branched alkyl group with 1 to25 C atoms which may be unsubstituted, mono- or polysubstituted byhalogen or CN, it being also possible for one or more non-adjacent CH₂groups to be replaced, in each case independently from one another, by—O—, —S—, —NH—, —N(CH₃)—, —CO—, —COO—, —OCO—, —OCOO—, —S—CO—, —CO—S—,—CH═CH—, —CH═CF—, —CF═CH—, —CF═CF— or —C≡C— in such a manner, that inthe whole molecule oxygen atoms are not linked directly to one another,or, in case they are not linked to an O atom, one or both of them may beH; MG²¹ and MG²² are each, independently of one another, a mesogenicgroup, SP² is a divalent spacer group comprising 1 to 40 C atoms, itbeing also possible for one or more CH₂ groups in the spacer groups tobe replaced, in each case independently from one another, by —O—, —S—,—NH—, —N(CH₃)—, —CO—, —COO—, —OCO—, —OCOO—, —S—CO—, —CO—S—, —CH═CH—,—CH═CF—, —CF═CH—, —CF═CF—, —CF₂— or —C≡C— in such a manner, that oxygenatoms are not linked directly to one another; i and j are, independentlyof each other, 0 or
 1. 15. A liquid crystal medium according to claim 1,wherein the dielectric anisotropy (Δ∈) of the liquid crystal medium isin the range of 60 to
 90. 16. A liquid crystal medium according to claim1, wherein the medium comprises the chiral dopant R-5011 or itsenantiomer S-5011


17. A liquid crystal medium according to claim 16, wherein theconcentration of the chiral dopant R-5011 or its enantiomer S-5011 inthe medium is from 4.0 to 5.0% by weight.
 18. A liquid crystal displaywhich comprises a polymer stabilized liquid crystal material accordingto claim
 6. 19. A liquid crystal display according to claim 18, whereinthe liquid crystal material is aligned in the uniformly standing helixmode.
 20. A process for the preparation of a composite comprising aliquid crystalline material and a polymeric material wherein the processcomprises polymerizing one or more reactive mesogenic compounds ofcomponent D in a liquid crystalline medium according to claim
 1. 21. Aliquid crystal medium according to claim 1, wherein the one or morepoly-reactive mesogenic compounds in component D include one or moredi-reactive mesogenic compounds.
 22. A liquid crystal medium accordingto claim 1, wherein component D includes one or more isotropic reactivecompounds.